Superconducting magnet coils have been wound with multistrand Rutherford-type cable for many years, and the advantages of such cable are well known. Rutherford-type cable was named after the Rutherford Laboratory in England which first produced such cable.
Rutherford-type cable is made by winding or twisting a plurality of superconductor strands around an elongated mandrel to form a semifinished generally cylindrical cable having a hollow core. Pressure rollers are then employed to flatten the hollow core cable into a flat multistrand ribbon having two generally flat sides and two edges.
The general practice has been to employ strands made of superconductor wire which is substantially round in cross section. The wire generally comprises superconductor material which is copper-clad, or clad with some other soft metal, to protect the superconductor material and to provide current carrying stability.
When the helically wound cable is flattened by the pressure rollers, the round external surfaces of the strands are flattened to a considerable extent. The pressure rollers cause the finished ribbon cable to have a compact cross section of known density.
However, the pressure rollers do not flatten the round interior surfaces of the strands to any substantial extent. Instead, the action of the pressure rollers produces an irregular indentation or embedment of the strands of one layer into the strands of the other layer at each point where each initially round strand crosses another round strand in the other layer of the cable. This indentation of the round strands in the core of the flattened cable produces interlocking knobs or knuckles which resist flexing or bending of the finished ribbon cable. This resistance to flexing may be called the knuckle effect. Any flexing or bending of the cable, or any change in tension, forces the internal knuckles out of register and results in irregular internal stresses between the knuckles within the cable. Thus, any flexing produces a low and irregular compressive group modulus across the thickness of the cable. Any flexing or movement of the strands along the edges of the cable produces an increase in the thickness at the point where each strand curves back to the other side of the cable. The result is to produce a cross section of a "dog bone" shape. This shape tends to create difficulties with the insulation required between turns and also results in a wound dipole magnet coil having ends which are not mechanically rigid.
When the cable is flexed or bent, the localized stresses between the knuckles may cause localized damage to the strands which tends to reduce the current carrying capacity of the cable, in that the damaged cable tends to lose super-conductivity and to revert into normal resistive conductivity at a lower current, than in the case of a cable which has not been damaged.